1. Field of the Invention
The present invention relates to a DC power source unit that can selectively charge a battery pack, which is used as a power source for a cordless power tool, and supply DC voltage to the cordless power tool through an adaptor shaped like the battery pack.
2. Description of the Related Art
Cordless power tools are convenient because they can be used anywhere without being restricted by the length of a power cable. However, whenever the battery runs low, the battery pack needs to be removed and charged or else replaced with another battery pack that has already been charged.
A DC power source unit that converts alternating current to direct current can be used along with the battery pack. That is, the DC power source unit can be used when the cordless power tool is to be operated in a single place that has a source of alternating current, and the battery pack can be used when the cordless power tool is to be operated at several different places that have no source of alternating current. However, this situation has a problem in that the operator of the cordless power tool must carry both the DC power source unit and the battery pack's charging unit to the work area.
Japanese Patent Application Publication No. 2000-184614(A) discloses a DC power source unit with a battery charging function. When the DC power source unit detects that the attached cordless power tool is being operated, it supplies power only to the cordless power tool and not for charging purposes. On the other hand, when the DC power source unit detects that the power tool is not being operated, then it charges a battery pack.
When a battery temperature gradient is used as a parameter to determine a full charge condition of the battery pack, an inaccurate determination of the full charge condition may result because the battery temperature gradient tends to lower during the charge interruption period.
Battery packs with nickel-cadmium or nickel-hydrogen batteries incorporated therein will reach a full charge condition in a short period of time if they are charged with a relatively large current. However, charging the battery pack with a large current generates a large amount of heat from the battery, thereby causing the cycle lifetime of the battery pack to shorten. In order to suppress generation of heat from the battery, it has been proposed to forcibly cool the battery using a cooling fan during charging. Forcibly cooling the battery allows a large current to be supplied to the battery. The battery packs to which the forcible cooling is applicable and the battery packs to which the forcible cooling is not applicable are different in structure. The former battery packs are formed with air inlet/outlet holes and the latter battery packs are not. Because there is little cooling effect in the battery packs with no air inlet/outlet holes, such battery packs are to be charged with a small current. In this manner, the battery packs are charged with different charging modes depending upon whether the battery packs are adapted for the forcible cooling.
When the battery temperature gradient is used as a parameter to determine the type of the battery packs, the same problems arise because the battery temperature gradient tends to lower during the charge interruption period.